The injection molding process used predominantly in industry today involves an intermittent process where: 1) a “shot” of polymer is melted; 2) two cooled mold halves are clamped together under a clamping force; 3) the “shot” of molten polymer is forced into the cooled mold cavity; 4) the polymer cools to a solid state; 5) the mold cavity opens; 6) the molded part is removed. This process is repeated to form multiple parts using the same mold cavity. Furthermore, this same process is used to produce multiple parts simultaneously, where multiple mold cavities are filled at the same time, in so-called multi-cavity injection molding systems.
This intermittent process has numerous limitations, including: 1) all processes occur in a sequential fashion, which lengthens the time required to mold a part since each step must be completed before the next step can begin; 2) to produce large quantities of parts it is necessary to have many mold cavities filled simultaneously—this requires very large equipment to hold the mold closed since the clamp tonnage must hold multiple mold cavities closed all at the same time, and the molds are very large to accommodate the multiple mold cavities.
One approach to address these issues is to “compression mold” molded articles. This approach involves: 1) extruding molten polymer; 2) trimming a “plug” of extruded polymer to a predetermined length (to achieve a target volume of polymer); 3) depositing the “plug” into a bottom mold cavity; and 4) compressing an upper mold half in to a bottom mold half to form a molded part. This approach can be accomplished on a continuous rotating platform which enables each step to be accomplished simultaneously, and results in very high production rates and lower costs. However, there are numerous trade-offs. First, the polymer “plug” freezes immediately when contacting the cooled bottom mold half—this results in a noticeable matte or rough surface texture on the molded part (an undesirable quality defect). Second, the molds must be very simple in design to enable the part to be molded by the compressive forces as the upper mold half approaches the bottom mold half—this dramatically limits the part designs that are possible using this molding technique.
An alternate approach is to continually feed the polymer to a plurality of mold cavities arranged in a carousel fashion about a central polymer source. In existing continuous injection molding systems of this nature that have been proposed or put into practice, it is understood that the mold cavities are disposed about the central polymer source in a planar, hub-and-spokes fashion, with the polymer source outlet or nozzle being in the same plane as the inlet of each of the mold cavities. One drawback of this arrangement is the large footprint of manufacturing floor space required to accommodate all of the mold cavities. Another drawback is the amount of energy necessary to propagate the polymer along horizontally-extending feed channels that connect the nozzle and the mold cavities. An additional drawback is the lack of ability to make real-time adjustments to melt pressure. In at least one prior disclosure of a carousel-type continuous molding system, the system had a valve gate actuator for positioning a valve pin that controllably connected the molding cavity to a shooting pot. The valve gate was operated according to a valve gate cam profile for actuation of the valve pin. Because the valve's actuation is dependent upon a cam track, the valve position is dictated by the location of a mold position as it rotates about the carousel. As such, there is no ability to adjust the melt flow to increase or decrease pressure. The only variable determining the rate and pressure by which melt flows into a given mold cavity is the extent to which the valve is open or closed, but with no ability to make fine adjustments at the location of the valve, any pressure adjustments that may be needed would have to be accomplished by adjusting the rate of output of an extruder or other source of molten polymeric material.